1. Field of Invention
The present invention relates generally to an improved circuit for an occupant restraint system for automotive vehicles such as an airbag system or a pretensioner for a seat belt, and more particularly to an energy reserve circuit designed to supply electrical energy to operate an occupant restraint system for a given period of time after the power supply from a main power source to the occupant restraint system is interrupted by a vehicle crash.
2. Description of Related Art
In a typical air bag system for automotive vehicles, an acceleration sensor monitors the acceleration of the vehicle. When it is determined that a vehicle crash has occurred based on the monitored acceleration of the vehicle, a squib is fired to deploy an air bag to protect an occupant from the impact.
A firing circuit activating the squib and a crash monitor are usually supplied with electric power from a storage battery mounted in the vehicle. A backup capacitor serves as an auxiliary power supply for the firing circuit and the crash monitor if electrical communication of the firing circuit and the crash monitor with the storage battery is blocked by the impact of the vehicle crash. The backup capacitor must have sufficient capacitance to store enough electrical energy to activate the squib for a given period of time after the power supply to the firing circuit and the crash monitor is blocked.
Thus, actuating the air bag system within a normal operating voltage range (e.g., 6 V to 16 V) requires a backup capacitor having a capacitance so large as to be impractical. In order to alleviate this problem, a conventional air bag system uses a step-up circuit which increases the voltage of the storage battery for charging the backup capacitor up to a given increased voltage (e.g., 14 V). This allows use of the backup capacitor having a smaller capacitance.
The conventional air bag system, however, has a drawback. The backup capacitor is usually connected directly to the firing circuit and the crash monitor. The step-up circuit is thus to designed to charge the backup capacitor up to the given increased voltage and to hold it for actuating both the firing circuit and the crash monitor if the battery voltage drops. Specifically, the step-up circuit is required to have the ability to output high-power (e.g., 14 V, 100 mA). Such a step-up circuit usually needs large-sized parts such as coils, which can counter attempts to miniaturize or reduce manufacturing costs.
In order to avoid the above drawback, a system taught in Japanese Patent First Publication No. 7-96815 uses two backup capacitors for a firing circuit and a crash monitor which are charged by a step-up circuit. A switching transistor is disposed between the backup capacitors. The backup capacitors are usually connected to each other by an on-operation of the switching transistor, and are disconnected by an off-operation of the switching transistor when the crash monitor outputs a firing signal to the firing circuit in the event of a vehicle collision. If the communication of the step-up circuit with a storage battery is blocked, the firing circuit uses the electrical energy stored in either of the backup capacitors to deploy an air bag.
The above prior art system, however, has at least two disadvantages; one, that the switching transistor does not contribute to reduction in output ability of the step-up circuit, and two, that the use of the two backup capacitors results in increasing component parts.